Method of enhancing the wetstrength of papers



y 1938- M. o. SCHUR 2,116,544

METHOD OF ENHANCING THE WET STRENGTH OF PAPERS Filed Aug. 4 1936Patented May 10, 1938 METHOD OF ENHANCING THE WET STRENGTH OF PAPERSMilton 0. Schur, Berlin, N. H., assignor to Brown Company, Berlin, N.H., a corporation of Maine Application August 4,1936, Serial No. 94,157

30 Claims. (Cl. 91-70) v This invention relates to a method of enhancingthe wet-strength of various kinds of papers, including substantiallywaterleaf papers and papers impregnated with colloidal binders of thenature of glue, casein, viscose, etc., which are introduced into thepaper as aqueous solutions or suspensions and which, upon drying of thepaper, are more or less irreversibly set to impart waterresistivity orhigh wet-strength to the paper.

I have discovered that the water-resistivity or wet-strength ofsubstantially waterleaf papers and papers impregnated with binders ofthe foregoing character may be enhanced to an amazing degree by theapplication of heat to the dry paper,

- and advantageously by the momentary applicadrier drums at temperaturesof about 212 to 265 F. While various temperatures of heating upwards ofabout 300 F. may be employed according to my process in theheat-treatment of the predried, substantially waterleaf orbinder-impregnated paper for the purpose of enhancing gogreatly itswater-resistivity or wet-strength, I have found it in many instancespreferable to work within a temperature range of about 400 to 650 F.,for instance, to expose the dry paper to a temperature of about 475 to500 F.,'as at such temperature it is possible to develop practicallyinstantly a phenomenal increase in the wetstrength of the paper withoutinjuring it and to do so inexpensively with a comparatively smallheating surface while the paper is traveling continuously and at highspeed, for instance, from v the dry end of a papermaking or impregnatingmachine. I

I shall now give instances of the application of my invention todemonstrate the remarkable and '45 advantageous results that can berealized thereby. In one case, my aim was to produce a paper of maximumwet-strength such as is of value in wrapping wet vegetables, such aslettuce and celery, or lining the crates in which such vegetables 50 iniced condition are packed and shipped. To this end, vI may impregnate awaterleaf paper sheet of any suitable basis weight and fiber compositionwith a glue solution containing about 3% glue, based on solution, andabout 2% formalde- 5 hyde, based on glue. The sheet may be squeezed uponemergence from an impregnating bath of such glue solution, as by squeezerolls, so that it retains about its own weight of the impregnatingsolution, whereupon it may be air-dried, say, at 70 to 100 F. When asheet of kraft pulp pa- 5 per of 47 pounds basis weight was thusimpregnated and dried as usual, it was found to have a bursting strength(Mullen) of 12 pounds per square inch after a thorough soaking throughwith water effected by its submergence in a water 10 bath for one-halfhour. When an impregnated and dried sheet of similar character was,according to the instant invention, exposed for roughly one second to atemperature of about 475 to 500 F., its wet-strength, that is, burstingstrength 15 after similar water-submergence, was 35 pounds per squareinch. This remarkable increase in the wet-strength of the paper sheet bymomentary intense heating was had with practicallyno burning or othersensible injury thereof. The heating 20 of the paper sheet for twoseconds at such temperature increased its wet-strength to 39 pounds persquare inch without significant burning of the fiber, but heating for alonger period did not enhance the wet strength of the paper materially25 and the paper became noticeably embrittled. When the paper wasexposed to a temperature of 435 F., it was found possible to increaseits wetstrengthmarkedly by extending the time of its onds. Animprovement may also be realized by exposing the paper to a temperatureof 350 F.

for about 1 to 2 minutes.

As already indicated, it is preferable in many cases to expose the paperto maximum safe ele- 35 vated temperature for a very short time. In thisconnection, it is to be borne in mind that heating of arapidly movingpaper sheet, for instance, a sheet traveling continuously at 200 to 400feet per minute from the dry end of a paper-making or W impregnatingmachine, requires extensive heat-. ing surface and that the provision ofsuch heating surface and its maintenance at temperatures upwards of 300F. is prohibitively costly. A oneminute heat treatment performed on asheet traveling at a speed of 200 to 400 feet per minute obviouslyrequires a heating surface 200 to 400 feet long, whereas, on the otherhand, a one-- second heat treatment reduces the length of the heatingsurface to 1/60th, that is, to only 3.3 to 6.7 feet. It is thusunderstandable why I prefer, especially in the case of high speed paperproduction, to "conduct the heat treatment at about 475 to 500 R, whichrepresent a temperature to which most papers can be exposed momentarilywithout being. materially injured, and to correlate such temperaturewith an area of heating surface and a paper speed corresponding roughlyto about a one-second heating period. In some instances, I may go to anelevated temperature as low as 400 F. or to an elevated temperature ashigh as 650 F., depending upon such factors as composition of paper,surface texture, basis weight, etc.

Another example involving the use of my invention was centered about theproduction of paper toweling possessed of good absorbency as well ashigh wet-strength. In such case, an absorbent, waterleaf paper sheet ofkraft pulp having a basis weight of 37 pounds was impregnated with a 2%aqueous solution of ammoniated casein, squeezed to a content ofimpregnating solution about equal to the dryweight of paper, and driedon the usual steam-heated drums at 240- F. The wet-tensile-strength of aone-half inch width strip of the resulting paper sheet averaged 1.5pounds.

In testing for wet-tensile-strength, strips one-half inch in width werepulled apart a few seconds after having been streaked transversely witha camel-hair brush wet with water.

Since the dried impregnated sheet is quite waterabsorbent, the teststrips become wet throughby the time the tensile load is appliedthereto. A

test similarly performed on the paper sheet before it was impregnatedwith a casein solution gave a wet-tensile-strength of 0.7 pound. Whenthe dried, impregnated sheet was heated for one second at 475 to 500 F.and for about two to three minutes at 350 F., its wet-tensile-strengthwas increased to 2.5 pounds. The presence of a small amount offormaldehyde in the impregnating bath may boost the wet-strength valuesboth before and after the shot of intense heat.

As already indicated, the short, high temperature heattreatment of myinvention may be advantageously applied to various binder-impregnatedpapers whose binder depends upon dehydration to render it relativelystrong when rewet. Thus, papers impregnated with viscose solutions orsuspensions and other aqueous binders, besides albuminous binders,appearing as wetstre'ngthening agents in paper may be made to exert amuch greater wet-strengthening effect on the paper by such heattreatment even after the paper containing such binder has already beendried thoroughly, as ordinarily. Of course, the surprising enhancementin wet-strength of the paper to be gained by my invention varies withthe different binders. The presence of formalde-' hyde or other tanningagent in solutions of glue, casein, or other albuminous binders, such asI may employ herein, which causes a tanning reaction on the binderduring the momentary intense heating of the binder-impregnated paper,may be advantageous in that such reaction may sometimes be brought morenearly to completion and the finished paper product ordinarily hashigher wet-strength than isotherwise the case.

It is to be understood that my invention may be advantageously appliednomatter at what stage of paper-making the binder is infused into thepaper sheet. Thus, in some instances, the glue, etc., may be introducedinto the bulk pulp or fiber preparatory to its sheeting on the papermaking machine; or such binders may be applied, as by spraying, to thewet or freshly -formed paper web before it reaches the dry end of thepapermaking machine, for instance, in advance of the last press rolls.In the examples hereinbefore given, thesubstantially dry, absorbentwaterleaf paper sheet as it issues progressively from the dry end of thepaper-mat ing machine may be passed progressively through a bath of thebinder solution, excess solution removed from the sheet immediately uponits emergence from .the bath, as by squeeze rolls or scraper blades,whereupon the sheet may be redried to completion and finally exposed toa temperature upwards of about 300 for less than temperature ofthe'drum, the extent to which the drum is wrapped by the paper sheet,and the speed of the sheet may be controlled to yield the best resultsaccording to the nature of the hinder, the wet-strength desired in thefinished paper product, and other circumstances of manufacture. The costof such a heating instrumentality is relatively low, for it is ofcompact simple construction and easily operated and controlled. In someinstances, it may be desirable to cool the paper before it is reeled oraccumulated or to moisten it with a small amount of water, say, withabout 5% to 7% of its own weight of water, but these expedients, beingsimply additive to the invention hereof and being readily accomplishedin simple ways by those skilled in the art, need not be discussed indetail or illustrated.

I While the invention hereof has been described as being applied topaper, it should be obvious that its utility may extend also tobinder-impregnated woven'fabrics and to carded or felted fiber webs, forinstance, tocarded cotton fiber webs or to wool felts wherein it may bedesired to attain the highest wet-strengths by the use ofbinder-impregnants of the class hereinbefore described. I shall use inthe appended claims the expression interfelted fibrous sheet material"as covering carded webs felts, and papers, as all such fibrous sheetmaterial is characterized by a more or less interfelted fibrousstructure or texture. So far as concerns the aspect of producing by myinvention papers of the highest wetstrength, it will, of course beappreciated that the resulting papers might be of such high bindercontent as to be "sized or water-repellent or, in the case of viscoseoralbuminous binders, such as glue and casein, be of such controlledbinder content as to be water-absorbing and hence adapted for such usesas toweling, handkerchiefs, diaper linings, etc. It is obvious that thepaper might be composed of various fibers or fiber mixtures,

for instance, of the usual chemical wood pulps, such as sulphite orkraft, wood pulps refined-to distinctly higher alpha-cellulose contents,say,

' upwards of 93%, rag pulp, etc.; and the paper base might be formedwith various degrees of absorbency, depending upon the use to which thefinal paper product is to be put. Indeed, the paper mightcontainnon-cellulosic fibers such as wool and asbestos. Should my invention beapplied to papers comprising esssentially only mineral fibers, such asasbestos, it becomes possible to heat the binder-impregnated paper attemperatures higher than 050 F. and for a. time longer g 2,110,544 thanmere seconds by reason of the incombustibility of such fiber. In-suchlatter case, the limit in temperature and time of final heat treatmentis essentially that at which the binder is injured or decomposed byheat.

It isv possible by applying the inventive principles hereof to produceimpregnated paper products of surprisingly high dry tear or shearresistance coupled with unusually high wetstrength (Mullen andtensile)Thus, I may start with refined wood pulp (Duracel) prepared by exposingkraft pulp to the refining action of comparatively strong alkalineliquor, for instance, so-calied white liquor, at about room or slightlyelevated temperature until the pulp has acquired an alpha cellulosecontent of about 90 to 92%. Such refined pulp in either unbleachedorbleached condition is put into substantiallyuniform aqueous suspensionpreparatory to the papermaking operation while preserving its fiberlength'as much as is compatible with the realization of the desiredsubstantially uniformly tax-- tured waterleaf paper sheet therefrom. Thehighly absorptive waterleaf paper sheet thus formed from such pulp isimpregnated with a glue solution containing about 3% to 4% glue solidsand a small amount of formaldehyde, squeezed to a glue content of about3%, based on the dry weight of paper, air-dried at about roomtemperature, and finally brought into momentary contact with anintensely heated surface, as hereinbefore described. The resulting paperproduct is characterized by its unique combination of remarkably highwet-strength and very high dry tear-resistance. It might be noted thatsuch a paper product is not only'similar to vegetable parchment in itshigh wet-strength and wet scuff-resistance but far surpasses ordinaryvegetable parchment in its toughness or dry tearresistance. Because ofthe comparatively low glue content of such a paper product, it retainsto a very large degree the flexibility of its waterleaf paper base asopposed to the comparatively stiff or brittle paper product producedwhen a similar paper base is impregnated to much higher glue contentwith a view toward giving it the high wet-strength of the paper producthereof simply by the usual passage over steam-heated drier drums of thepapermaking or impregnating machine. The high dry tear-resistance of thepaper product hereof is a reflection of the preservation of high averagefiber length and fiber flexibility in the pulp entering into itswaterleaf paper base; and the glue impregnant brought to the desiredhigh degree of irreversibility by the intense heat treatment hereof isrelied upon herein for the purpose of binding together the fibers anddeveloping in the paper the wet-strength ordinarily sought to be gainedby parchmentizing the fibers and thus sacrificing verysignificantiy thetear-resistance in the paper product.

The invention hereof may be applied advantageously to waterleaf papers,that is, papers whose fabrication is carried out without the addition ofbinder to the paper-making stock at any stage of papermaking. Thus, bysubjecting various substantially waterleaf papers to the hotshottreatment hereof, it is possible to increase and appended claims, I meana paper fabricated in the complete absenceof binder or sizingingradientor containing binder or sizing ingredient of such type or in such smallamount that when made as heretofore its wet-strength is markedly lowerthan the percentage ratios just expressed; andI'do not mean to includeby such quoted expression papers which have received a special chemicalhydrating treatment, such as parchmentization. The hot-shot" treatmentmay in the case of such substantially waterleaf paper involve subjectingsuch paper, after it has been dried in the usual way, to a one-secondheating on each face by progressively passing the paper sheet as it iscoming from the papermaking machine in contact with a surface heated toabout 450 to 500 F. In this way, the wet-strength (tensilestrength of astrip wide) of a waterleaf sheet of kraft towellng was increased from aninitial value of 0.64% to 1.0#; of a waterleaf sheet com posed of 50%kraft and 50% groundwood from an initial value of 0.6# to 1.2#; of awaterleaf sheet composed of refined wood pulp and intended for nitratingpurpose from an initial value of 0.3# to 1.0#. Results of a similarorder were obtained with various other papers, for instance, papers madefrom Duracel pulp, sulphite pulp, and cookedmanila pulp. I shall nowgive in tabular form various characteristics of particular kinds ofpaper before and after a hot-shot treatment as above described.

Tests before hot-shot Mullen Tensile (p (i /l4) Tm Watrleiii papers DryWet Dry Wet Dry Wet 45f Kraft (106% Ostrand"). 48 1. 3 13. 6 1. 1 102 264 DuraceP' 52 1. 7 12. 7 1. 0 157 42 40* Alpha N. 13 1. 0 5. l D. 3 7312 Tests after hot-shot Mullen Tensile (po (i /a") Waterleaf papers DryWet Dry Wet Dry Wet 45* Kraft (100% Ostrand 48 5. 0 13. 6 2. 2 98 58 48#Duracel'krfl- 52 5. 4 14. 4 2. 1 128 82 40# "Alpha N" 14 l. 3 4.? 0. 967 24 All test specimens of paper were soaked in water for one-half hourbefore the various wet tests were applied thereto; and the wet Mullentest was determined by bursting through 4 plies of the material underexamination and dividing the gage reading by 4.

In other examples involving the application of the invention hereof,small quantities of binder, including casein and rosin size, were addedto the papermaking stock after the manner of making the usual so-calledengine-sized papers. Thus, A% casein, based on the dry weight of fiber,was precipitated from an ammonia-casein solution by means of alum. in apapermaking stock of 50% kraft and 50% groundwood. The stock wasconverted on a paper-making. machine into a sheet of creped toweling of37# basis weight. When dried in the usual way, the paper toweling had awet-tensile-strength of 0.6#, but when subjected after drying to contacton each of its faces for one second with a surface at about 450 to 500F., the wet-strength of the paper toweling was increased to 2.5#. Theresulting paper toweling was very absorbent and was not embrittled toany serious extent by the momentary heating. A similar improvement inthe quality paper both in waterleaf and tub-sized condition before andafter the hot-shot treatment above described.

' Tests before hot-shot f I Mullen Tensile (points) #m") 5'. we

. scuff rating Dry Wet Dry Wet Dry Wet 4MRom8l"krafi a1 4.0 10.0 1.3 68it. 8 Same paper tub-sized with sodium caseinate with deposition of 1%casein (based on weight of paper) 39 5.0 11.5 1.6 76 92 8 Tests afterhot-shot Mullen Tensile T ints r Wet (D (/36 m rating Dry Wet Dry WetDry Wel- #Romal"krait. 38 14.3 10.4 3.3 62 99 30 Same paper tub-sizedwith sodium caseinate with deposi-. tionil 1% Caste!!! ase on we 0 1From the foregoing tables, it is apparent that the present inventionvmakes possible in a simple and inexpensive way a marked and importantincrease in the utility of ordinary papers. Apropos of such increasedutility, it might be noted that ordinary wrapping paper, for instance,is enhanced by-the invention hereof tremendously in its'resistance tobreakage or scuff when thoroughly soaked with water and ishencefi-endered much more suitable as a wrapping paper which maybe'soaked, as by exposure to rain, or for fabrication into bags intendedto hold moist or wet vegetables, or for use as aerate-liner, etc.

The present invention thus makes possible among.

other things the production of paper products adapted' to supersede invarious fields of use requiring papers of high wet-strength suchcomparativelyexpensive prior art paper products as parchment papers, waxpapers, etc. t So far asI know, I am the first to recognize thatwaterleaf papers and'ordinary sized papers, for instance, rosin-sizedwrapping paper, may beconverted by the high temperatureheating treatmenthereof into finished paper products whose resistance to thedisintegrating effect of' water is enormously greater than that of theoriginal papers. The finished waterleaf paper products hereof have, asalready indicated, a Mullen wetstrength of more than about 8% of the dry'Mullen strength and a wet tensile strength of morethan about 12% of thedry tensile strength. The papers sized with rosin and heat-treated ashereinbefore described have a wet Mullen strength or wet tensilestrength higher than about 25% of the corresponding dry tests; and suchhighwet-strength is inherent in the heat-treated paper base itself forthe sizing agent in the heattreated paper product having such high wet!strength may be one for instance, rosin size, that has but little or nowet strength itself on long soaking of the paper in water and that henceimparts very little or no wet-strength to the paper. In other words, thefunction of the sizing agent itself may be simply and essentially cose,casein, etc., and to sized papers whose size content does not contributesignificantly to the high wet-strength of the finished paper producthereof, it appears likely that in the case of all of these papers thereis an incipient' caramelization or searing of the fibers and/or of thecolloidal matter present in the paper under the high temperature heatingtreatment hereof and that such action is accompanied-by the-formation ofwet-strengthening reaction products in the paper base; or it is possiblethat wet-strengthening reaction products arise in the paper baseas-aresult of some oxidation of the fibers and/or of the colloidal matterunder the high temperature heating treatment hereof. Whether or not theforegoing explanation of the mechanism underlying the wet-strengtheningaction in paper realized by the invention hereof is'correct, it is a.fact that I am enabled pursuant to the present invention to produce atlow expense very valuable paper products heretofore unknown to theindustry,

so far as I am aware.

I claim:

1. A method of producing. a. paper product of high wet-strength, whichcomprises exposing a predried' paper sheet containing,.if at all, only avery small amount of binder to temperatures upwards. of about 300 F. forsuch a limited period of time as to avoid sensible burning of or injuryto such sheet while markedly increasing its wetstrength.

' 2. A method of producing a paper product'of' high wet-strength as adry paper sheet is being fabricated progressively at high speed, "whichcomprises progressively bringing the dried sheet into only momentarycontact with a surface heated to about 400 to 650 F.

v 3. A method of producing substantially waterleaf paper ofliighwet-strength, which comprises exposing a predried'sheet of the waterleafpaper to temperatures upwards of about 300 F. for such alimited periodof time as to avoid sensible burning of or injury to such sheet whilemarkedly increasing its Wet-strength.

4. A method of producing sized paper of high wet- -strength, whichcomprises exposing a predried sheet of sized paper containing only avery small amount of size to temperatures upwards of about 300 F, forsuch a limited period of time asto avoid sensible burningof or injury tosuch sheet while markedly increasing its wet-strength. 5. In a methodinvolving the impregnation of fibrous sheet material with aqueous bindercomposition whose binder content tends to set irreversibly-upon thedrying of such sheet material, that step which comprises exposing thedried impregnated sheet material containing only a very small amount ofthe binder to temperatures upwards of about 300 F. for such a limitedperiod of time as to avoid sensible burning of or injury to such sheetmaterial while markedly increasing its wet-strength.

6. In a method involving the impregnation of fibrous sheet material withaqueous albuminous 5 binder composition and drying such sheet material,that step which comprises exposing the dried impregnated sheet materialcontaining only a very small amount of such binder to temperaturesupwards of about 300 F. for such a limited period of time as to avoidsensible burning of or injury to such sheet material while markedlyincreas-- ing its wet-strength.

7. In a method involving the impregnation of an interfelted fibroussheet material with aqueous binder composition whose binder contenttends to set irreversibly upon the drying of such sheet material, thatstep which comprises exposing the dried impregnated sheet materialcontaining only a very small amount of the binder to temperaturesupwards of about 300 F. for such a limited period of time as to avoidsensible burning of or injury to such sheet material while markedlyincreasing its wet-strength.

8. In a method involving the impregnation of an interfelted fibroussheet material with aqueous albuminous binder composition and drying.such sheet material, that step which comprises exposing the driedimpregnated sheet material containing only. a small amount of suchbinderto temperatures upwards of about 300 F. for such a limited period oftime as to avoid sensible burning of or injury to such sheet materialwhile markedly increasing its wet-strength;

9. In a method involving the impregnation of a 85 fibrous base withaqueous albuminous binder composition and drying such base, that stepwhich comprises exposing the dried impregnated base containing only asmall amount of such binder momentarily to a temperature of about 400 to650 F. i

10. In a method involving the impregnation of a fibrous base withaqueous albuminous binder composition and drying such base, that stepwhich comprises exposing the dried impregnated base containing only asmall amount of such binder momentarily to a temperature ,of about 400to 650 F. in the presence of an insolubilizing agent for said albuminousbinder.

11. In a method involvingthe impregnation a paper sheet with aqueousbinder composition whose binder content tends to set irreversibly upon.the drying of the impregnated sheet ,7 and involving further theproduction of the dried impregnated paper sheet progressively and athigh speed, that step which comprises progressively exposing the driedimpregnated sheet containing only a small amount of such bindermomentarily to intense heat and thereby to enhence thewet-strengththereoi.

12. In a method involving the impregnation of a paper sheet with aqueouscolloidal binder composition whole binder content tends to setirreversibly upon the drying of the impregnated sheet and involvingfurther the production oi the dried impregnated paper sheetprogressively and 'at high speed. that step which comprisesprogressively exposing the dried impre nated sheet containing onlyasmall amount or such binder so to a temperature upwards of about 300 1".for suchslimitedperiod of timesstoavoidsensible burning of or injury tosuch sheet while markedly increasing its wet-strength.

13. In a method involving the impregnation of a fibrous sheet withaqueous binder composition whose binder content tends to setirreversibly upon the drying of the impregnated sheet and involvingfurther the production of the dried impregnated sheet progressively andat high speed, that step which comprises progressively bringing thedried impregnated sheet containing only a small amount of such binderinto only momentary contact with a surface heated to about 400 to 650 F.

14. In a method involving the progressive impregnation of aprogressively moving paper sheet with aqueous albuminous binder solutionand the progressive drying of the impregnated paper sheet, that stepwhich comprises progressively bringing the dried impregnated sheetcontaining only a small amount of such binder to a very high temperaturefor such a limited period of time as to avoid sensible burning of orinjury to such sheet while markedly increasing its wetstrength.

15. In a method involving the progressive impregnation of aprogressively moving paper sheet with aqueous albuminous binder solutionand the progressive drying of the impregnated paper sheet, that stepwhich comprises bringing the dried impregnated sheet containing onlyasmallamount of such-binder into momentary contact with a surface heatedto about 400 to 650 F. and thereby enhancing the wet-strength impartedthereto by said albuminous binder.

16. In a method involving the progressive impregnation of aprogressively moving paper sheet with aqueous albuminous binder solutionand the progressive drying of the impregnated paper sheet, that stepwhich comprises exposing the dried impregnated sheet containing only asmall amount of such binder .in the presence of an insolubilizing agentfor said albuminous binder momentarily to a temperature of about 400 to650 F. and thereby enhancing the wet-strength imparted thereto by saidalbuminous binder.

17. A paper product which has been exposed in dried condition to atemperature upwards of about 300 F. for such a short period of time as19. A sized paper product which contains only a very small amount ofsize and which has been exposed in dried condition to a temperatureupwards oiabout-300 Fdor such a short period or time as to besubstantially unscorched while being improved markedly in itswet-strength.

20. A method of producing a paper product.

of high wet-strength, which comprises exposing a. predried paper sheetcontaining, it stall, only a very small amount of binder to temperatures0! about 400 to 650 1". for such a limited period of time as to avoidsensible burning of or injury to such sheet while markedly increasingits wetstrength.

. MILTON O. SCHUR.

